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ASTRONOMERS DISCOVER FREE-FLOATING PLANETS IN THE ORION NEBULA

Last Updated on Saturday, 08 May 2010 14:28

Published on Tuesday, 22 February 2005 00:00

The most sensitive survey ever undertaken of the region in the Orion Nebula where new stars are forming has revealed 13 "free-floating planets" as well as more than one hundred very young brown dwarfs. The discovery was made by Dr Philip Lucas of the University of Hertfordshire and Dr Patrick Roche of the University of Oxford using a new camera on the United Kingdom Infrared Telescope (UKIRT) in Hawaii. Their results will be published in the Monthly Notices of the Royal Astronomical Society.

Brown dwarfs are objects that might have become stars, but never accumulated sufficient material. With less than 8% of the Sun's mass, they did not heat up enough inside to trigger the nuclear reactions involving hydrogen that keep stars shining over long periods. Nevertheless, they do produce some nuclear energy for a short time (from deuterium, a rare isotope of hydrogen) if their mass exceeds 1.3% the Sun's mass - about 13 times the mass of Jupiter. Astronomers regard this as the minimum mass for a brown dwarf.

The new infrared survey of the Trapezium Cluster in the Orion Nebula, turned up 13 objects below the 13 Jupiter-mass threshold. The mass of the smallest is equivalent to no more than about 8 Jupiters. These objects have been dubbed "free-floating planets". They give off only residual heat left over from when they were born. By nature they are more like the giant planets of our solar system than stars. However, they do not orbit any star and drift through space by themselves. Only two similar objects have previously been discovered. (Japanese astronomers found them in the southern Chamaeleon Nebula.) The discovery of thirteen more in one cluster suggests that they might be very common.

Astronomers believe that most stars are born in giant molecular clouds - vast clumps of cold gas and dust. The nearest such cloud lurks just behind the glowing gas of the Orion Nebula. The Trapezium cluster at the heart of the Orion Nebula has recently broken out of the dark molecular cloud. It is therefore the best place to look in order to find out about the creation of stars, brown dwarfs and free-floating planets in the rest of the Galaxy. The backdrop of the Orion Molecular Cloud obscures everything that lies behind it, which is very useful because it means that all the objects seen in this part of the sky are members of the cluster, except for perhaps a handful which lie in the foreground.

Because brown dwarfs and free floating planets quickly cool down, they are easiest to find when they are young and still retain some heat from the formation process. The objects in the Trapezium cluster are mostly about one million years old - very young compared to the five-billion-year age of the Sun.

An interesting feature of this study is that no planets have been found under 8 Jupiter masses. It may indicate that there is a limit to how small these free-floating planets can be but even more sensitive surveys will be needed to confirm this. In the meantime UKIRT has been used to obtain spectra of about twenty of the brown dwarfs and planets. The results are still being analysed but they show the signature of water vapour that is expected in relatively cool stars and brown dwarfs, at a temperature of a mere 2700 degrees Centigrade. The planets will eventually cool down to earthly temperatures but it is unlikely that they could ever sustain life. Although the total number of brown dwarfs and planets in the Trapezium may be similar to the number of stars, individually they have less mass. If this a typical cluster, brown dwarfs and planets do not contribute significantly to the dark matter that many astronomers believe pervades the universe.

This survey is one of the first projects undertaken with the new infrared camera UFTI, the UKIRT Fast Track Imager. It is the most sensitive search yet conducted for low mass stellar and sub-stellar objects. For all the objects they detected, Lucas and Roche measured the strength of radiation given off at three standard wavelengths in the near infrared (known as I, J and H). They used this data to deduce the mass, luminosity and temperature of the objects. UFTI was built by a team headed by Roche and Lucas at Oxford University in 1998, with the help of several British universities.

ILLUSTRATION

A low-resolution JPEG version is attached. This, and higher resolution versions may be downloaded from the following web pages:

Caption: An infrared picture of the central part of the Orion Nebula constructed from the three separate images taken with UFTI (the UKIRT Fast Track Imager) on the United Kingdom Infrared Telescope. The three colours used in this false colour image (blue, orange and red) correspond to infrared radiation with wavelengths twice as long. Hence, this picture shows Orion as we would see it if our eyes were sensitive to light of twice the wavelength they are actually capable of detecting. The picture shows stars, brown dwarfs and planets together with diffuse starlight, which is scattered by tiny particles of cosmic dust, and light emitted by energised gas. Infrared images are vital for this work, as they penetrate the dusty clouds and are able to pick up faint objects which cannot be seen in visible light images.

NOTES1. Both UFTI and UKIRT are funded by the Particle Physics and Astronomy Research Council.

2. The paper describing this work has been accepted for publication in the Monthly Notices of the Royal Astronomical Society, but the date of publication and is not yet available.